peat bags in the filter will make your water a bit more acidic however as
soon as the peat is exhausted then the PH will climb back up.

It will only climb back up as a result of water changes with harder water.
Treating water with peat (in or out of the tank) basically drops the Kh. A
lower Kh gives a lower Ph at a constant level of CO2 i.e. it basically
removes the buffer from the water and its a permanent change.

If things are
working well now why bother changing it?.

Indeed

I would do away with the carbon insert which after a few days just becomes
a bag that acts as a mechanical
filtration and add another sponge or some bio max rings.

Agreed.

I.